Particle origin when particle is a group
Alternative 01. If you want a whole group to behave as one leaf.   duplicate the leaves ... then [Join] the new meshes into one single mesh.  You can change the origin of a single mesh in edit mode.





Alternative 02. Deselect [Whole Group] if you want your leaves separate.





When this is done increase the number of particles and bring image in 3D View up to date and inspect.  Increase or Decrease the number to suit your needs.